1. Field of the Invention
This invention relates to record player turntables and particularly relates to devices for dampening, dissipating, and blocking vibrations and resonances that interfere with faithful sound reproduction.
2. Review of the Prior Art
Playback distortion from the turntable frequently occurs in even the finest equipment. It may be an obvious distortion which makes listening very unpleasant, an objectionable resonant coloration, a blurring of clear, distinct sound into an unrecognizable mass of sound, a subtly annoying but not totally unpleasant effect, or even an unidentifiable source of fatigue. Mechanical vibration in a turntable may originate in, or be transmitted by, the drive system, the loader assembly, the platter design, or the chassis design.
Feedback is a major source of mechanical vibration which may be either mechanical or acoustical. Mechanical feedback is energy transmitted through the floorboards and to the wall beams and the like so that the loudspeaker is mechanically coupled to the turntable. Acoustical feedback is created by acoustical energy emanating from the loudspeaker or other sources when it moves or pumps energy into the room in the form of low-to-high level pressures at multiple frequencies and in complex patterns and with changing forces. A mechanical force is thus created when the pressure patterns in the air are absorbed by solid objects.
The lower the frequency, the more obvious the mechanical force becomes until it reaches a frequency too low to be heard. But even at such low frequencies, sufficient energy can be absorbed to rattle windows and shake walls as well as to create mechanical energy in the turntable platter, its main board, its base, and its supporting structure. Each of these parts vibrates with its own characteristic resonances in accordance with varying amounts of acoustical energy in the room.
These mechanical and acoustical vibrations travel through the equipment and coincide from all directions at certain key pathways to the tone arm. The result of such combination seems to be a compounded increase in the feedback to the tone arm at many key points which might be called "collision course vibrations". These collision course vibrations are also generated within the mechanism itself, by and between the motor and the main bearing and the chassis and the subchassis, and are transmitted to and picked up by each end of the tone arm.
Such vibrations are commonly measured in the laboratory as rumble. Rumble is a low-pitched vibration or frequency that is caused by a mechanical vibration acting on the turntable and tone arm when the vibration occurs at the rotation frequency of the motor, the idler, the bearing, or the platter, or at some multiple of any of these frequencies. The platter bearing is indeed the main source of rumble in turntables that are now available on the market. Rumble may be reported as weighted or unweighted. Weighted rumble measurements discriminate against subsonic frequency components which cannot be reproduced by loudspeakers or heard by the human ear, but such frequencies can overdrive an amplifier or speaker and thereby impair the reproduction of higher frequencies. Thus, an unweighted measurement can also provide useful information because both sonic and subsonic frequencies--from one to 100,000 cps--contribute undesirable side effects.
Flutter is a rapid pitch fluctuation in reproduced music which is caused by pulsations or changes of the turntable speed, i.e., a rapid variation from constant rotational speed. When flutter occurs at a low rate, it is called "wow", suggesting the characteristic sound it imparts to steady musical tones. When it occurs at higher rates, the effect is of a "gargling" or roughness. Wow and flutter are usually reported as a combined flutter measurement which is weighted to emphasize the most objectionable flutter rates at around 5-10 Hz. This combined flutter measurement is usually specified in hundredths of a percent of perfect accuracy with 0.03% being a typically good figure.
Flutter robs a musical instrument of its character by blurring the musical image. Flutter can be characterized as forward and backward movement. The composite of all of these vibrations creates a situation that has much the same distortional effect, with respect to playback, as flutter itself but with more severe characteristics because these vibrations react in all planes and in 360.degree. C.
Even though such laboratory measurements report excellent values, such as an average peak wow and flutter of not more than .+-.0.03% and a rumble low enough to produce an ARLL-weighted measurement of -73 db or even -80 db, collision course vibrations can produce annoying disturbances to the trained ear. Neither is consequently acceptable for quality equipment.
Numerous devices have been designed and built for decoupling the turntable from mechanical vibrations. However, the frequency at which the energy decoupler resonates must be above the rotational speed of the turntable, which at 331/3 rpm is approximately one-half cycle per second, and at the same time, must be lower than the resonant frequency of the tone arm mass and cartridge compliance, which is preferably 8 or 9 cycles per second. Thus, the best frequency for decoupling mechanical energy to the turntable is two or three cycles per second. Such decouplers include the use of a dense, thick, massive support board, upon which the turntable is placed, and the use of a number of coil springs between the support board and the platform therebeneath. Adding additional mass to the frame of the turntable also changes its frequency of vibration and reduces distortion that may range from frizzy highs to muddy lows, e.g., that is, music "out of focus".
About twenty years ago, a turntable having excellent acoustical qualities is believed to have been advertised. This turntable featured a centering pin that expanded for precisely centering a record and remained in place during play.
U.S. Pat. No. 1,163,566 teaches the use of a layer of sponge rubber, which is secured to a fabric stretched over the revolving platter, and upon which the record is placed.
U.S. Pat. No. 1,823,533 discloses sound deadening materials such as cork washers which surround the central shaft and support the platter.
U.S. Pat. No. 2,353,763 shows an apparatus for supporting the driving motor on vibration insulating washers between coil springs and attaching the turntable shaft to the turntable with a soft rubber washer and an oil-retaining felt washer beneath an annular cap.
U.S. Pat. No. 3,214,177 described a dampening and isolating means in combination with a horizontally mounted, ball-type, flush-type, anti-friction bearing which reduces both lateral and vertical rumble vibrations from the turntable to -80 db below 0 level. This disclosure points out that modern sound pick-up means, enormously amplified by modern, extremely high-gain, wide-frequency range amplifiers, are superimposed with the reproduced sound as turntable rumble sound vibrations which modulate with and hence impair the clarity of the reproduced sound so as to be extremely disturbing to the listener. While using dampening washers above and beneath the mounting plate and between the mounting plate and the spindle, turntable rumble was found to become practically inaudible when tested with the prescribed RIAA equalization for a test record.
U.S. Pat. No. 3,058,790 teaches the elimination of wow by use of neoprene washers for minimizing vertical movement of the turntable.
U.S. Pat. No. 3,984,112 discloses the use of a synthetic elastic rubber-like plastic material as a mat which covers the top of the platter and its edges, being moldably fitted thereto so that the cover mat is secured to the platter without using an adhesive which can deteriorate. The record is supported on the mat by a pair of annular ridges. However, this mat would not isolate the record from platter resonances.
U.S. Pat. No. 3,997,174 describes a flat hollow-mat containing a visco-elastic fluid which imparts vibration-dampening properties and accomodates warped records.
U.S. Pat. No. 4,012,048 describes a recording disc having concentric rings of soft rubber and a central circular section of an acoustic dampening material such as soft butyl rubber which functions as an adapter and insulates the record from the spindle.
With all of these improvements, Audio of June 1977 reported on page 38 as follows:
"Many attempts have been made to isolate or decouple the turntable mechanically from the room via springs or rubber mounts. Some have been moderately successful, others are a sham. Only in a very few cases have they been totally successful in properly decoupling the turntable from mechanical vibrations, and most have been unsuccessful in removing the previously described acoustic feedback problems."
There is accordingly a need for a simple, generally applicable, and efficient decoupler for collision course vibrations.